Pulled back to Earth in the DGIV??

[destinos]

Hey all,

I was making a routine climb into orbit last night to try and tackle the Earth to Mars tutorial in Go Play In Space.

I was climbing along a heading of about 42 degrees. For most of the flight my pitch was higher than the recommended 30 degrees, at some point in the climb my VV started falling, dipped below the horizon, and i found myself somehow being pulled back to earth even though my nose was up and main thrusters were as maxed as they have ever been.

Does anyone understand the mechanic that would have caused this?

My guess is that in the winged craft, I failed to generate enough ground speed before leaving the part of the atmosphere that generates altitude-sustaining lift. So it's possible to have too steep an ascent in such a craft.

If this is right reasoning, does anyone know whereabouts the atmosphere thins to a point where lift becomes negligibly weak?

 

[MJR]

You either stalled or have unlimited fuel on.

 

[Zatnikitelman]

Though I'm not sure if the DGIV has any thing special about its ascent profile, I'd recommend pitching to around 70 degrees to get out of the atmosphere fast, then pitch back down. A good rule of thumb is to maintain 60-70 degrees pitch till you're at 50km then pitch down to 50 degrees. I typically only watch the pitch profile until I'm at this point, then I start watching my vertical speed and acceleration. Once I've got my Apogee where I want it, I adjust my pitch up and down to maintain that Apogee.

 

[MJR]

Use trim too. It helps out a lot without losing speed.

 

[the.punk]

You must fly an very high angle. I had too this problem.
60-70 degree should do it until your 40-50 km how Zatnikitelman said.
Then you can gain horizontal speen.

 

[Tommy]

A DGIV gets it's maximum lift at an AoA of about 17 degrees. If you are losing altitude and have a pitch of 15 degrees, it means you are going to slow for your altitude.

Maintaining a high pitch up to 40k will work for a non-scram vessel (or one with no wings), but it's not very efficient.

There are three forces that need to be overcome to attain an Earth orbit - Inertia, Gravity (vertical) and Drag (mostly horizontal). There are two forces at our disposal - Lift and Thrust. Inertia is overcome only after the other two forces (Gravity and Drag) are overcome and exceeded.

The high pitch angle technique mentioned above uses the thrust to overcome both Gravity and Drag, and doesn't really use the wings much. A lower pitch angle means the thrust vector is more aligned against the horizontal drag force, and the wings provide the Lift to overcome Gravity. This means the fuel is used more for gaining orbital velocity, and less to gain altitude. The end result is a more fuel efficient ascent.

That being said, Drag forces vary with altitude and speed. It decreases with altitude and increases with speed. This means it's best to keep the speed low until you are out of the really thick part of the atmosphere (the first 10 to 15 k) to keep the Drag low. It pays to get above the thick air a soon as possible to minimize the fuel needed to overcome the high drag.

Put this all together, and you come up with an "optimized" ascent profile for winged vessels such as the DGIV, stock DG, XR series, etc.

After takeoff, reduce throttle to hold around 200 - 250 m/s velocity while you retract the gear and turn into your launch heading. Once the correct heading has been attained, pitch up to about 70 degrees and go full throttle. You shouldn't be going supersonic until you are over 10k - if you do it means you should use a higher pitch during this initial ascent phase.

Once over 10k alt, use about 3/4 downward trim to reduce the pitch. You should be flying more or less level by 25k alt. From here maintain about 200 m/s vertical speed until you are over 33k alt, then slowly reduce it down to about 100 m/s by about 40k.

You should achieve a suitable Ap of about 210k by the time you get to 65 - 70k, and have a velocity of around 7k or better. Remember to establish your Ap a bit (5k or so) higher than you want as you'll lose a bit due to drag. Cut thrust when the Apoapsis is at a suitable altitude, and coast up to it. Then burn prograde to raise your Periapsis and circularize your orbit.

This approach is more fuel efficient, by having the correct speed for the altitude we get more force from Lift than we lose from Drag.

For Mars simply skip the initial high pitch phase, and start with a target of 200 m/s vertical speed.

For Titan, maintain the high pitch until over 100k, and keep the throttle down to keep airspeed around 250 max until over 100k. Essentially, a Titan ascent profile is very much like an Earth profile, but multiply the Altitudes by 10.

 

[destinos]

Tommy, thanks a bunch for the careful and thoughtful analysis. Highly informative and certainly worth a try.

Have you watched Dr. Schweiger's playback on his Earth to Moon tutorial? The approach he uses is to pitch up to 30deg until he hits around 25k and then he slowly pitches down to 20deg. Then, as his VV starts to fall, his tactic seems to be to vary his AoA somewhere between 5 and 10deg above the VV's changing position for the rest of the ascent. It's a pretty graceful insertion, but probably less fuel efficient than what you've described.

 

[Tommy]

I've noticed that Drag increases pretty fast at over 6 -7 degrees AoA, so I try to maintain a low enough altitude at any given speed so that I can maintain sufficient lift at about 5 degrees AoA. However, I haven't done a lot of experiments yet to determine the absolute best profile. A lower initial AoA may be better, but so far it seems that getting over 10k as quickly as possible works better. I've tried using a lower pitch at first, and even throttling down to keep the airspeed subsonic, but it still ended up using more fuel than the 70 degree initial pitch.

 

[RisingFury]

I usually go vertically up until I'm at 10k, then I start pitching down. By the time I'm at 20k, I drop my velocity vector to 10° and adjust AOA to hold that.

The reason is that at 20k, you're above most of the atmosphere and thus drag becomes small... I use as much lift as I can, together with engine thrust to keep gaining altitude. I usually reach orbital velocities by the time I'm at 70k.

 

[TMac3000]

Is it possible you forgot to engage the turbo-pump? It's very hard to take off without it.

 

[the.punk]

I always take off without turbo pump.

 

[RisingFury]

Is it possible you forgot to engage the turbo-pump? It's very hard to take off without it.

Not really... DeltaGliderEX has far less thrust then weight, yet it can get to orbit.

 

[destinos]

Is it possible you forgot to engage the turbo-pump? It's very hard to take off without it.

I...don't believe I've ever engaged anything before taking off. are we talking about the dg-iv here? Is this some kind of prank? Where are the cameras? Where's Ashton??

 

[MJR]

The turbo pump provides 60Kn of power depending on the engine after takeoff. It is on the lower panel with the engine controls.

 

[Oceanic]

wow tommy thanks for the very handy information, i tried several approaches using the Mark-IV engines and found that your approach (without using the Turbopump) saves ~7.5% main fuel against the pro903specNN autopilot.

 

[Tex]

Very well written post there Tommy. I tried your approach and it worked good for me too. I only use the DGIV autopilot when I'm feeling lazy and don't want to manually fly into orbit as it does waist a lot of fuel climbing straight up for too long.

 

[Tommy]

I did some comparative studies on ascent profiles for a tutorial I'm working on. It will cover all the elements of LEO operations - ascent, plane changes, sync, et, and have comparative studies of the various ways to accomplish each task. For instance, I'll be comparing four different ways to make a MIR - ISS plane change to demonstrate the different approaches and the economy of each. Don't hold your breath, though. It will be a few months before I get this done.

 

[destinos]

Tommy, i've used the method you detail to get to orbit in the DG and i can make it work consistently now, but I never seem to make it up there with more than about 42-45% fuel remaining. Does that sound about right? I would think it possible to get to orbit on less with the DG.

 

[Oceanic]

Tommy, i've used the method you detail to get to orbit in the DG and i can make it work consistently now, but I never seem to make it up there with more than about 42-45% fuel remaining. Does that sound about right? I would think it possible to get to orbit on less with the DG.

i had about 49% fuel remaining with the DGIV (Mark-IV engines and 2.8t payload) launching from Wideawake heading 118° altitude was 250km and ecc near 0

 

[Tommy]

A stock DG can get into a 210k parking orbit from the pad at Canaveral (Hover) with 55 - 60 % fuel left (7550kg). For the ISS alt, a bit less, but I'd say you're still missing something. You may be using the hovers more than needed, or simply going to high to soon. Your VS can start as high as 200 when you're around 25k, but it should be down under 150 by 40k, and around 100 by 60k. You want to have a velocity of over 7500 by the time you hit 70k, with MECO soon after as soon as the apoapsis is high enough (about 200k). Apo may be as much as a quarter orbit ahead, just keep about 5 degrees AoA until over 125k, then engage prograde.

During the ascent, keep an eye on the AoA displayed by SurfaceMFD. It you need an AoA over 7 degrees to maintain altitude, you are too high. Set the trim fairly nuetral, maybe one click "up", and let yourself sink down a bit. As your speed increases you will start to rise again, and keep the VS under 100 for a bit.